A Dynamic Thermal Camouflage Metadevice with Microwave Scattering Reduction

Abstract With rapid development of radar and infrared (IR) surveillance technologies, the need for microwave‐IR compatible camouflage is now more than ever. Here, a novel multispectral metadevice is proposed to simultaneously achieve microwave scattering reduction, dynamic IR camouflage, and low IR reflection. This metadevice is constructed by the coding thermoelectric elements with the properly designed phase arrangement, and the incident microwave energy can be redirected to the nonthreatening directions for specular reflection reduction. The dynamic IR camouflage with low IR reflection is realized by using the thermoelectric cooling and heating effect and high‐IR‐absorptivity surface. The above three functionalities are demonstrated by experimental measurement. The 10 dB scattering reduction can be realized at the microwave band of 10–16.1 GHz. In the IR region, the designed metadevice can not only dynamically modulate the surface temperature for matching different background temperatures, but also realize the pixel temperature control for adapting to a spatially varying thermal background. In addition, it reflects almost no surrounding thermal signals compared with the traditional low‐emissivity IR stealth material. This study paves an effective way to achieve microwave‐IR compatible camouflage, which may inspire the future researches and applications in multispectral camouflage and stealth fields.


Microwave Scattering Reduction
Liming Yuan, Cheng Huang, Jianming Liao, Chen Ji, Jingkai Huang, Yuetang Wang, and Xiangang Luo* S1. Simulated results of the microwave reflection performance for the TEMD elements under different incident angles.
Figure S1a illustrates the simulated cross-polarized reflection characteristics of the '0' and '1' TEMD elements under linearly-polarized illuminations with the different incident angles. It can be seen that the binary elements have the cross-polarization reflectance of more than 0.9 over the frequency range from 10.2 to 16.0 GHz for all the incident angles. Meanwhile, the relative difference between the cross-polarized reflection phases of the binary elements under different incident angles is close to 180 in most of the above frequency bands, as shown in Figure S1b.

S2. Thermal simulation for the TEMD element.
Heat is exchanged between the target and the surrounding through the thermal resistances of the upper side, TEMD and the lower side. The thermal resistance of the lower side includes the thermal contact resistance (R TC ) at the interface between the TEMD and the target.
Because of the existence of micro-scale unevenness of solid surface, the real contact area is smaller than that of the nominal contact surface area, as shown in Figure S2, resulting in a significant increase in R TC , which could be expressed as follow   where the number '1' and '2' represent the bottom substrate of our TEMD and the target, respectively. In our thermal simulations, the values of , m, P, H C and the convection coefficient at the air side are 1 m, 0.4, 100 kPa, 3 Gpa and 25 W m −2 K −1 , respectively. The spacing distance may lead to non-uniform temperature distribution. For this purpose, we investigated the surface temperature distribution of our TEMD element under the identical average temperature for different spacing distances. In the simulation, the average surface temperature of the TEMD element was set to be 10℃ higher and lower than the temperature of the target (30℃) at the heating and cooling modes, respectively. The Simulated air-side surface temperature distribution is illustrated in Figure S3. It can be observed that the higher the surface temperature is, the larger the maximum variation is, i.e., the maximum temperature variation at the heating mode is larger than that at the cooling mode for the same TEMD element. At the heating mode, the maximum temperature variation changes from 1.26℃ to 1.16℃, while it slightly changed from 0.32℃ to 0.29℃ at the cooling mode, as the spacing distance increased from 3.65 mm to 4.65 mm. The above result shows that the

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Gas Gas Target   4 spacing distance in our design has slight influence on the surface temperature uniformity, which may provide more freedom to optimize the microwave scattering reduction performance.

S3. Microwave scattering simulation of the designed TEMD
The full-wave simulation model of the designed TEMD is composed of 44 subgroups distributed in the chessboard-like configuration. Each subgroup is made of 44 basic elements, i.e., the '0' or '1' element, to ensure the electromagnetic similarity between the subgroup and the corresponding basic element. The TEMD is illuminated by the normal linearly-polarized wave at the frequency band of 10~17 GHz. Simulated results of the scattering patterns of our TEMD are displayed in Figure S4. Here, the scattering magnitude is normalized by the PEC plate with the same size. As expected, the incident microwave is mainly redirected into four symmetrically oriented directions at the effective response frequency, resulting in greatly suppressing the specular scattering. The direction (deflection angle , azimuth angle ) of the anomalous scattering waves can be calculated by the following equations: in which,  is the wavelength of the incident wave; L x and L y are the periodic lengths along xand y-direction, respectively. For the constructed model, L x = L y = 8p = 52 mm. It can be seen that the scattering elevation angle at 15 GHz is about 30, which is consistent with the theoretical calculation (33) by Equation (1). As shown in the figure, it could be seen that the more high-efficiency polarization conversion the binary elements has, the greater reflection reduction our TEMD can obtain.  Figure S5. The simulated specular reflection of the TEMD is given in Figure S5b, which also includes the same-size PEC plate for comparison. It is noted that the TEMD with the optimized phase distribution can achieve the 10-dB scattering reduction over a wide frequency band of 8.5~16.5 GHz, except for the frequency of around 11 GHz where about 9-dB scattering reduction is obtained. Figure S5c~f show the 3D scattering patterns of the TEMD at the four representative frequencies of 10 GHz, 12 GHz, 14 GHz and 16 GHz, respectively. It is found that the TEMD can diffuse the backscattering wave to all directions, and the scattering energy in each direction is very small, achieving significant reductions in both monostatic and bistatic RCSs.

S4. Experimental demonstration for the microwave reflection reduction
The microwave reflection characteristic of the TEMD sample is measured by using the arch 7 measurement system, as seen in Figure S6. Two linearly polarized standard horn antennas respectively set as receiver and transmitter are placed on an arch range and the sample is located at its circle center. Figure   We also measured the backward scattering pattern of our TEMD sample in the 45 plane, as shown in Figure S8. It can be seen that the backscattering wave is redirected into the predesigned directions and then the specular reflection is sharply suppress. The measured results agree with the simulated ones in general. Some deviation between them is mainly caused by the fabrication tolerance and the measurement error. In addition, the measurement environment also has some difference from the simulation condition, for instance, the ideal plane wave in the simulation is difficult to be realized by the horn antenna in the experiment. GHz, respectively.

S5. Study on the surface temperature modulation
The cooling and heating effects of our TEMD are characterized by aid of a thermocouple and a thermostat stage, as shown in Figure S9a,b. The thermostat stage is fixed at a constant temperature, and the TEMD surface temperatures under the different applied currents are recorded by the thermometer. A DC source is utilized to supply our TEMD sample current.
The bias voltages (V) along with the corresponding currents (I) were also recorded for extracting the electrical resistance (R), as shown in Figure S9c. Then the consumption power (P) can be calculated by P=I 2 R under various applied currents, as shown in Figure S9d.  The transient temperature change was recorded in the measurement for the heating mode under the external current of -0.11 A and the cooling mode under the external current of 0.14 A, respectively. As Figure S11 shows, it takes about 100s for our TEMD to reach the target temperature in both heating and cooling modes.

S6. The demonstration of the pixel temperature control
Here, we supplied the different bias currents to the pixels of the letter 'I', 'O' and 'E'. Details on the loaded currents are given in Figure S12. It can be seen that the IR image of each letter display the gradient colors. This result further demonstrates that our TEMD has the pixelmanipulation capability of controlling the surface temperature.

S7. Fabrication process of our TEMD
As illustrated in Figure S13, the process of fabricating our TEMD mainly includes the with the electrode, they are pull out from the mold, and then they are covered with the top carrier substrate, on which, the electrode has been coated by the solder paste through the same process described in the second step. Finally, the top carrier substrate is heated to solder the TE pillars with the electrode to finish the fabrication of our TEMD sample. Figure S13. Fabrication process of our TEMD.